Exhaust gas filters for Diesel engines have the purpose of filtering soot out of the exhaust gas of Diesel engines. The emission of soot from Diesel engines is critical because the soot particles contain adsorbed at their surface small amounts of substances some of which in high concentrations are considered to be harmful to human health.
The soot filters for the Diesel exhaust gases conventionally consist of a monolith of a porous ceramic material which is constructed of thin-walled ducts that are parallel to one another and extend over the whole length of the monolith in the flow direction of the exhaust gases. These ducts are alternately closed at their upstream or downstream end. The ducts are frequently open or closed in the manner of a chessboard pattern, so that each duct has an open and a closed end. Another way to make filters of this type consists of winding the filter in a spiral shape in the manner of manufacturing a corrugated board, to form a gas-permeable packet. In this case, the individual ducts are no longer developed only in the manner of a chessboard, but are also arranged in a spiral shape.
When the Diesel exhausts are led through a filter of this type, the exhaust gas entering through the ducts that are open on the inlet side is forced to pass through the porous duct walls and flow into the adjacent ducts that are open on the outlet side. In this case, the soot particles that can no longer pass through the duct walls collect in the ducts that are open upstream. When a certain final ignition temperature is reached and when a sufficient oxygen concentration exists in the exhaust gas, the collected soot burns off and the filter is regenerated. However, it was found that the porosity of the conventional soot burning filters decreases as times goes on, so that a rising backpressure builds up in front of the filter leading to a reduction of power in the engine and an increase of fuel consumption.
The objective of the present invention consists of finding a process for manufacturing an exhaust gas filter for Diesel engines made of a porous ceramic material whereby the decrease of porosity as well as the rising counterpressure in front of the filter that occur over the working life of the filter can be reduced considerably.
This objective is achieved by means of having a multi-zoned filter having different porosities in each zone, as well as by a process for manufacturing an exhaust gas filter whereby the filter is partially immersed in a solution or suspension of a porosity-reducing agent to create the above zones.
The filter block contemplated in the invention consists of several zones along the length of the filter, where the porosity of the duct walls in each zone decreases from zone to zone in the main flow direction of the gases from the gas inlet to the gas outlet. In each case, plugs are alternately arranged at the border of the zones. On the side of the gas inlet, every other duct is closed by a plug. The gas enters into the ducts that are open, which ducts are subsquently closed by plugs at the border to the next zone which has a decreased wall porosity. As a result, the gas is forced to pass through the porous wall of the first zone having walls with a relatively large wall porosity and thus arrives in the second duct, which duct is subsequently closed by a plug at the start of the next zone which has a further decreased wall porosity. As a result, the gas flow is forced to pass through the less porous walls of the second zone. It is now possible, after the passage through two zones with different porosity, to let the gases out of the filter. It is also possible to connect additional zones with correspondingly reduced porosity so that the gas flow is filtered several more times through walls having a decreasing porosity. It was found in this case that the best results are achieved when the gas flow must pass through two to four zones with different porosity. In this case, the expense for the manufacturing the filters is disproportionately high.
The staggering of porosities should be carried out in such a way that in the last filtering step through the wall having the least porosity, and sufficient gas passage is possible without excessive backpressure. A staggering of porosities has been especially successful where, in the flow direction of the gases, each successive zone is decreased to about 70 to 90% of the porosity of the preceding zone, in which case the porosity of the last zone normally should not be below 40%. While the porosity of the ceramic material in the case of the conventional filters is between 35 and 55% with an average pore diameter of about 10 to 35 micrometers, in the case of the present filter, the porosity of the first filter step is selected to be larger, approximately in the range of a porosity of 50 to 75%, with a correspondingly larger average pore diameter.
An especially simple process for the manufacturing of this filter consists of starting with a conventional filter block blank, the porosity of which corresponds to the porosity of the first filter step and thus represents the largest used porosity. Starting with the outlet end of the filter, this filter block will now be immersed up to the level of the first zone in a solution or suspension of an agent which reduces the porosity and then dried. The consistency or concentration of this agent must be selected to be such that, after the drying or possible required burning-in, the desired porosity of the part of the filter that was immersed is obtained. The porosity reducing characteristics of the corresponding agents must be able to determined easily beforehand by means of tests. If the filter is to have more than two zones, the filter may be immersed again, up to the level of the additional zone border, in the same or a different suspension or solution. The use of the same solution is suitable when the filter is dried before the second immersion, while the use of another solution is advantageous when the filter, without any intermediate treatment, i.e., still moist from the first immersion process, is subjected to the second immersion process. It may also be necessary to immerse the filter several times in a corresponding suspension or solution, if the desired reduction of porosity in a zone cannot be obtained by means of one immersion process.
All solutions and suspensions of porosity-reducing agents that can withstand the later operational temperature of the filter block and are correspondingly abrasion-proof with respect to the gas flow may be used. For this purpose, only essentially inorganic substances can be used. Because they are easy to handle, water glass solutions have proven to be especially useful. In the case of higher demands on the temperature stability, slurries or suspensions of materials resistant to high temperatures were also found to be useful, especially materials that are also used for the manufacturing of the filter, such as cordierite or china clay/feldspar/quartz mixtures. If necessary, these suspensions may also contain an inorganic binding agent, such as a small quantity of water glass. It is advantageous to adjust the concentration of the suspension or solution in such a way that with a single immersion, the desired reduction of porosity by 10 to 30% can be achieved after the drying and possible burning-in of the blank.
As soon as the filter block blank is provided with the desired number of zones having duct walls with decreasing porosity along the length of the filter in the flow direction, the corresponding alternately arranged plugs, at the filter inlet, at the zone borders and at the filter outlet, may be arranged in the ducts. At the filter inlet and at the filter outlet, this takes place by conventional methods. Inside the filter at the zone borders, this placement may take place by pressing out a pasty sealing mass consisting, for example, of a mass of talc and water glass, and inserting the mass into the filter duct at the corresponding zone border by means of a hollow needle, thereby closing the duct at this point.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.